Jacob Rees-Mogg claims the Treasury is 'fiddling the figures' on Brexit

Leaked analysis this week said Britain would be worse off economically under every Brexit scenario.

Tory MP Jacob Rees-Mogg claims Treasury officials are "fiddling the figures" in order to try to keep Britain in the EU customs union.

Leading Tory Brexiteer Jacob Rees-Mogg has stepped up his attack on the Treasury, accusing officials of "fiddling the figures" in order to try to keep Britain in the EU customs union.

As senior ministers prepare to discuss Britain's future relationship with the EU, Mr Rees-Mogg, the chairman of the influential pro-Brexit European Research Group of Tory MPs, said any deal which kept the UK in a common customs area with the EU would be unacceptable.

He claimed Treasury economic modelling suggesting Britain would be worse off in any arrangement outside the customs union was "clearly politically influenced."

His intervention came after Theresa May, on the final day of her official visit to China, appeared to leave the door open to some sort of customs agreement with the EU - even though ministers have said Britain will leave the existing customs union.

However, Mr Rees-Mogg made clear any such arrangement would be unacceptable to Conservative Brexiteers as it would prevent the UK from striking free trade deals with other countries.

"We need to be free to do deals with the rest of the world. We must be out of the protectionist common external tariff which mainly protects inefficient EU industries at the cost to British consumers," he told the BBC Radio 4 Today programme.

He also repeated his controversial claim made in the House of Commons that he had heard the head of a pro-EU think tank saying Treasury officials had deliberately created an economic model to show that all options other than remaining in the customs union were "bad."

"Mr Grant said the Treasury is determined to keep us in the customs union. He also said on another occasion that the Treasury is 'newly emboldened'," he said.

"He is getting private briefings from the Treasury against Government policy. This is very serious. It is not for officials to invent policy.

"This is saying that they are determining policy, that they have an aim to lead policy against what the politicians may decide."

He said that since the decision to call the EU referendum in 2016, it was clear that the Treasury's economic forecasting had become politicised.

"With all forecasts, the assumptions you make at the beginning determine the outcomes that you get," he said. "If you look at the forecasts the Treasury made before the referendum, they were a humiliation. They were clearly politically influenced.

"With the referendum and with the EU, the Treasury has gone back to making forecasts. It was politically advantageous for them in the past. It is the same for them now.

"So yes, I do think they are fiddling the figures."

Mr Grant rejected Mr Rees-Mogg's account of his comments, although he acknowledged that many in the Treasury did regard staying in the customs union as the "less damaging option" for the economy.

"Mr Baker, a very honourable man, has apologised to me and to the House of Commons for saying things that weren't quite right. I am surprised that Mr Rees-Mogg hasn't apologised," he told Today.

"The Treasury cares about economics so it is naturally pushing for the sorts of Brexit that minimise the economic damage."

A Treasury spokesman told the BBC: "The prime minister and the chancellor have said repeatedly that we will be leaving the single market and the customs union. Any suggestion to the contrary is simply false."